In automotive engineering, for example, individual units are connected by a bus system (ISO 14230 Road Vehicle Diagnostic System) such as a data transmission line (K line, KWP 2000/Key Word Protocol 2000) over which the units communicate with an external automotive tester.
One example of such a unit is a vehicular airbag system, which has a control unit to deploy airbags in a motor vehicle if necessary on the basis of collision signals. The bus system connected to the control unit for this purpose has a diagnostic bus to detect any malfunction of the respective unit and to display the results.
To initialize or trigger a control unit connected to a diagnostic bus, the signals of the communication bus forming another part of an overall bus system containing the diagnostic bus are analyzed over the received data line or RxD line (RxD stands for received data line) of a microcontroller contained in the control unit. This is done on the basis of an initializing word or trigger word, in particular a 5-baud trigger word in two ways:
1) Sampling for high/low logic state is performed in a predetermined sampling cycle of preferably 10 ms. The corresponding information is read directly out of a port register (parallel interface register) of an interface between the control unit and the diagnostic bus.
2) The receive buffer register is read out and its status information is analyzed. Input is entered into the receive buffer register as soon as there is a transition from the high logic signal level to the low logic signal level on the bus, with the sampling of diagnostic information being performed by the bus interface receiver itself.
Initialization of the diagnostic bus achieved in this way (shown schematically in FIG. 2) is not reliable enough because there is no guarantee that the control unit connected to the diagnostic bus will be initialized or triggered unintentionally, e.g. by communication data from the tester or other stations.